Conventionally, in a railroad train, display devices using a liquid crystal display are arranged in each vehicle of the train as a display unit of an in-vehicle information system to inform guidance such as travel information and advertisements to the passengers (see, for example, Patent Document 1).
In such in-vehicle display devices, quality needs to be maintained constant to reliably inform the guidance such as travel information and advertisements to the passengers. Meanwhile, a liquid crystal display of the display device has an operating life, and its luminance decreases according to the time of operation. When the luminance of the liquid crystal display decreases, a screen display becomes dark to degrade display quality and image visibility becomes poor. Further, because the decrease in luminance of liquid crystal displays is not uniform and is individually different, maintenance needs to be performed for each display device.
Conventionally, therefore, a maintenance worker goes around a train to confirm display brightness (luminance) of display devices one by one, and determines necessity of maintenance such as replacement for each of the display devices.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-70254